C. F. Wheatley, Jr. and G. M. Dolny provide a general background description of the COMFET in their article "COMFET--The Ultimate Power Device; A General Study of Power Devices" on pages 121-128 of the November 1985 issue of SOLID STATE TECHNOLOGY. This article is incorporated herein by reference.
The COMFET has an insulated gate control electrode, which reduces the power required of the input circuitry, generally allowing that circuitry to be simplified. However, the source-to-drain circuit is like the emitter-to-collector circuit of a bipolar transistor, permitting conduction at current densities of several hundred amperes per square centimeter while forward voltage drop from source-to-drain is only a volt or so. This low forward voltage drop is available at high current densities because of the presence of minority carriers which conductivity modulate the high-resistance epitaxial layer forming the anode region. Such a device offers only medium switching speeds (in the order of fractions of milliseconds) with a switch-off tail caused by slow minority carrier recombination. Furthermore, there is an undesirable tendency for latch up to occur in a parasitic silicon-controlled rectifier in the COMFET structure.
To reduce the power loss during switching, it is desirable to switch the COMFET on and off at as high speed as possible. Faster switching is obtained by driving the gate of the COMFET from a voltage source with low internal resistance, so the RC time constant of that resistance times the gate capacitance of the COMFET is shortened. At these higher switching speeds the COMFETs undesirably show a tendency towards latch-up and consequent loss of control by voltage applied to the gate electrode. Unless the energy available to the COMFET during latch-up is carefully limited, latch-up results in destruction of the device.